Purification of steel

ABSTRACT

A process of essentially removing the aluminum oxide inclusions from molten steel which has been aluminum-deoxidized. The process comprises the addition to the aluminum-containing molten steel of from about 0.01 to about 0.05 percent by weight of magnesium based on the weight of the steel.

United States Patent [191 Frawley et al.

[451 Aug. 28, 1973 PURIFICATION OF STEEL [75] Inventors: James J. Frawley, Albany; Silvio S.

Gras, Schenectady, both of N.Y.; Louis D. Tote, Erie, Pa.

[73] Assignee: General Electric Company,

Schenectady, NY. 22 Filed: Mar. s, 1971 [2]] Appl. No.: 122,141

[52] US. Cl. .L 75/58 [51] Int. Cl. C2lc 7/02, C2lc 7/06 [58] Field of Search 75/53, 134 S, 58,

[56] References Cited UNITED STATES PATENTS 3,467,167 9/1969 Mahin..; 75/58 2,988,444 6/1961 Hurum 75/53 3,215,525 11/1965 Sprankle 75/134 S X 3,575,695 4/1971 Miyashita et a1. 75/58 3,291,655 12/1966 Gill et a1. 148/36 3,269,828 8/1966 75/58 3,131,058 4/1964 Ototani 75/58 1/1964 Contractor et a1. 75/58 Primary Examiner-L. Dewayne Rutledge Attorney-William C. Crutcher, Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forinan [57] ABSTRACT 6 Claims, No Drawings PURIFICATION OF STEEL This invention relates to a process for purifying steel by essentially removing the aluminum oxide inclusions in steel which has been deoxidized with aluminum.

A very important step in steel-making processes is deoxidation. During this step, small additions of deoxidizers, such as silicon, titanium, or aluminum, are added to the molten steel to lower the amount of dissolved oxygen and thereby prevent a chemical reaction between the carbon and oxygen during solidification. Most of the potent deoxidizers added to steel form a solid oxide inclusion when they react with the. dissolved oxygen. The integrity of the steel casting is dependent on the number, type, and distribution of these inclusions. For the best mechanical properties, it is desirable to have as few a number of possible of these inclusions per unit volume of metal and to have the inclusions evenly distributed throughout the metal matrix. Therefore, the reaction products formed during deoxidation should be such that they tend to coalesce and float out of the molten metal. If the reaction products do not rise to the surface, the number of nonmetallic inclusions will be large and the steel will be considered dirty.

The use of a combination of deoxidizers to lower the oxygen content and the inclusion content is known. For example, in the case of silicon deoxidation, if both sili con and manganese are added, the steel is cleaner than if silicon alone is used. The melting point of the complex oxide formed between silicon, manganese, and oxygen is lower than each individual oxide and below the temperature of the molten steel. The reaction product, which is therefore liquid, will then agglomerate, forming larger particles capable of rising out of the melt in a reasonable length of time.

Aluminum is widely used as a deoxidizer for steels and for many applications is superior for this purpose. However, the aluminum oxide inclusions which are formed are seriously detrimental to the quality of the steel. Additives are known which modify the deleterious effect and more uniformly distribute the aluminum oxide inclusions. However, it has heretofore proven difficult to remove the aluminum oxide.

It has now been found that the addition of magnesium to aluminum-deoxidized steel is an effective method of controlling and removing aluminum oxide particles from the melt. The magnesium is believed to combine with or fluxes the aluminum oxides, forming a more complex oxide. This causes the inclusions to coalesce and rise out of the molten metal faster than the small inclusions.

The invention is useful with a wide variety of steels including plain carbon, stainless, alloy and high temperature steels. It is particularly useful with high temperature ductility steels of the type covered by U.S. Pat. No. 3,291,655, assigned to the present assignee. Generally, from about 0.02 percent to 0.1 percent of aluminum, based upon the weight of the steel, is used as an inoculant for deoxidation of the steels. If less than about .02 percent aluminum is used, the amount of aluminum oxide formed will not normally require removal. In practice, approximately 0.04 .08 percent Al is used for deoxidation of the high ductility steels to which the present invention is particularly directed.

As little as 0.01 percent by weight of magnesium may be used, although virtually any amount of magnesium will reduce aluminum oxide inclusions and therefore produce improved results. Over about 0.05 percent of magnesium will not produce further improvement. Generally, the amount of magnesium used should accordingly range from about 0.01 to about 0.05 percent, and even more preferably from about 0.025 to about 0.05 percent. The magnesium may be added in almost any form but preferably as an alloy to slow down the dissolution and the reaction rate. Suitable magnesium alloys are, for example, nickel-magnesium, ironmagnesium, or aluminummagnesium. It is preferred that the magnesium alloy be added after the aluminum deoxidation step but it may also be added simultaneously with the aluminum during aluminum deoxidation. Generally, the magnesium alloy should be added to the ladle prior to tapping the furnace. The temperature of the melt at this point is normally between 2,850 to 2900 F., although this will vary of course with the specific alloy.

The addition of magnesium in accordance with the invention should be distinguished from the addition of magnesium to iron for the production of spheroidal cast iron. In the latter case, there is no problem with gas porosity for which aluminum or other deoxidizers must be added to kill" or calm the steel to avoid a carbonoxygen reaction. The magnesium is added to cast irons to change the graphite morphology. It acts as a nucleating agent to produce graphite in nodular or spheroidal form. The purpose, function and result of the magnesium addition to cast irons are totally dissimilar from those of the present invention.

A number of small test castings were prepared of aluminum-deoxidized steels. These small castings had previously been found to contain many of the defects observed in large production castings which were aluminum-deoxidized. They were accordingly believed to offer a useful means for obtaining comparative results. The steel alloys were melted in an induction furnace to the desired chemical composition and then deoxidized with aluminum. Two of the three samples were then inoculated with a nickel-magnesium alloy. The metal was poured in an air atmosphere into a silica sand mold. After cooling, the castings were sectioned .and test specimens were taken from the center of the castings. These specimens were mounted and polished, after which the numbers and types of inclusions were observed and their location marked. The inclusions were chemically analyzed, using an electron microprobe for qualitative analysis and X-ray diffraction for determination of crystal structure.

The following Table I shows the compositions of the three cast steel specimens. Sample 1 was not inoculated. Samples 2 and 3 were inoculated with 0.2 percent by weight of a nickel-magnesium alloy. The alloy contained 15 percent Mg or 0.03 percent of Mg based on the weight of the steel. All three steels were deoxidized with aluminum.

TABLE I C S Si Mn Cr N1 M0 V Ti Al 18 Fe Sam is .16 .01 .84 .64 1.35 .1 .04 .24 .006 .07 .0004 Hal. 2 .16 .01 .84 5 1.3 .2 .98 .24 .042 .08 .0006 Hal. 8 .12 .012 .46 4 1.49 .4 .08 .10 .028 .068 .001 Bel.

Sample 1 disclosed clusters or clouds of aluminum oxide of very small size, approximately 2 X inches in diameter. Samples 2 and 3 showed a considerably reduced number of inclusions and no evidence of the aluminum oxide clusters. These tests indicated that magnesium combines with the aluminum oxides, or fluxes with the aluminum oxides, to form a more complex oxide. This fluxing action causes the aluminum oxide inclusions to coalesce and rise out of the molten metal faster than the smaller inclusions in the cloud type. These complex oxides were identified by X-ray diffraction as of the type MgO:3Al O which is a solid solution between spinel and alpha-alumina. It should be noted that the aluminum values in Table I were measured using a spectrometer which measures both dissolved aluminum and aluminum oxide. Therefore Sample 1 has a higher aluminum content than Samples 2 and 3 because it has a higher aluminum oxide content. The starting aluminum contents in all three samples cover ranges which have included aluminum oxide clouds in steels which werent inoculated with magnesium.

The above indicates that with aluminum deoxidation, the steels contain clouds or clusters of aluminum oxide particles and the steel is what may be considered dirty. With the addition of magnesium to the aluminumdeoxidized steel,'the oxide morphology is effectively controlled; The aluminum and magnesium form a complex oxide, the oxide floats to the surface and the result is a cleaner, higher integrity steel casting.

We claim:

1. A process of essentially removing the aluminum oxide inclusions from molten bath of steel which has been inoculated with an aluminum-deoxidizing agent comprising adding to the molten bath of steel from about 0.01 to 0.05 percent by weight of magnesium based on the weight of the steel whereby a complex magnesium-aluminum oxide is formed and allowing said complex to float to the surface of the bath.

2. The process of claim 1 in which the magnesium is added as an alloy.

3. The process of claim 1 in which the steel has the following composition in percent by weight:

Carbon 0.05-0.6 Chromium 0.5-3.0

Molybdenum 0.3-1.75 Vanadium 0.l5-l.0 Manganese 0.2-1.5

Aluminum 0.02-0.2 Titanium 0.04412 Balance: Iron 4. The process of claim 1 in which the steel has been deoxidized with from 0.02 0.1 percent by weight of aluminum.

5. The process ofclaim 1 in which from 0.025 0.05 percent by weight of magnesium is added.

6. The process of claim 5 in'which the magnesium is added as a. magnesium-nickel alloy. 

2. The process of claim 1 in which the magnesium is added as an alloy.
 3. The process of claim 1 in which the steel has the following composition in percent by weight: Carbon 0.05-0.6 Chromium 0.5-3.0 Molybdenum 0.3-1.75 Vanadium 0.15-1.0 Manganese 0.2-1.5 Aluminum 0.02-0.2 Titanium 0.04-0.2 Balance: Iron
 4. The process of claim 1 in which the steel has been deoxidized with from 0.02 - 0.1 percent by weight of aluminum.
 5. The process of claim 1 in which from 0.025 - 0.05 percent by weight of magnesium is added.
 6. The process of claim 5 in which the magnesium is added as a magnesium-nickel alloy. 